1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus having a function of correcting nonuniform components of a static magnetic field and a static magnetic field correction method for use with the magnetic resonance imaging apparatus.
2. Description of the Related Art
The magnetic resonance imaging (MRI) requires very high uniformity in the static magnetic field in various imaging methods. The so-called shimming is important to correct nonuniformity in the static magnetic field. The shimming is roughly divided into passive shimming and active shimming. The passive shimming is adapted to make uniform the static magnetic field in an imaging region by placing shims in the static magnetic field generated by a static field magnet. The active shimming is adapted to make uniform the static magnetic field in the imaging region by superimposing correction magnetic fields generated by shim coils upon the static magnetic field generated by the static field magnet. In the recent magnetic resonance imaging, the active shimming is important. In the description which follows, we shall describe the active shimming and refer to it simply as shimming.
The nonuniformity of the static magnetic field can be represented by dividing it into components, such as zeroth-order components X0, Y0 and Z0, first-order components X1, Y1 and Z1, and second-order components X2, Y2, Z2, XY, ZY and ZX. There also exists higher-order components, such as third-order components, fourth-order components, etc.
In general, the shimming is performed for each of the nonuniform components described above. Since a shim coil is needed for each of the components to be corrected, correction is generally made on a limited number of components, such as components through first-order components or components through second-order components.
For shimming calculations, a collection of signals is actually made to measure a magnetic field distribution in a region of interest, then the resulting magnetic field distribution is expanded into magnetic field components set as objects of shimming, and correction magnetic fields are determined which cancel out the nonuniform components of the static magnetic field. To generate these correction magnetic fields, a current value to be applied to a respective one of the shim coils, i.e., a correction value, is calculated. Assume, for example, that the reference value of the strength of a uniform magnetic field is B0 and shimming is performed on the basis of a magnetic field distribution including components through first-order components. For more simplicity, assume that only the Z direction is considered in the expanded expression of the magnetic field distribution. Then, from an expression, C1Z+C0+B0, resulting from expansion of the magnetic field distribution into zeroth- and first-order components, the correction value for the first-order component is given as −C1 and the correction value for the zeroth-order component is given as −C0.
A conventional technique relating to shimming is disclosed in U.S. Pat. No. 4,740,753.
Since the current which can be set up on each shim coil has an upper limit, if the shimming correction value exceeds the allowable upper limit, proper correction will not be made. Even in this case, since the magnetic field distribution is expanded into components, the shimming correction values could be determined; however, since these values are the results of calculations under constraints of allowable current values of the shim coils, determined correction magnetic fields might not conform to the actual magnetic field distribution.